This invention relates generally to free-tipped axial-flow fans, which may be used as automotive engine-cooling fans, among other uses.
Engine-cooling fans are used in automotive vehicles to move air through a set of heat exchangers which typically includes a radiator to cool an internal combustion engine, an air-conditioner condenser, and perhaps additional heat exchangers. These fans are generally enclosed by a shroud which serves to reduce recirculation and to direct air between the fan and the heat exchangers.
The fans are typically injection-molded in plastic, a material with limited mechanical properties. Plastic fans exhibit creep deflection when subject to rotational and aerodynamic loading at high temperature. This deflection must be accounted for in the design process.
Although some engine-cooling fans have rotating tip bands connecting the tips of all the blades, many are free-tipped (i.e., the tips of the blades are free from connection with one another). Free-tipped fans are designed to have a tip gap, or running clearance, between the blade tips and the shroud barrel. This tip gap must be sufficient to allow for both manufacturing tolerances and the maximum deflection that may occur over the service life of the fan assembly.
Often free-tipped fans are designed to have a constant-radius tip shape, and to operate in a shroud barrel which is cylindrical in the area of closest clearance with the fan blades. In other cases, the tip radius is non-constant. For example, U.S. Pat. No. 6,595,744 describes a free-tipped engine-cooling fan in which the blade tips are shaped to conform to a flared shroud barrel. In either case, a significant tip gap is required, typically between 1 and 1.5 percent of the fan diameter.
Although tip gap will always reduce fan efficiency and increase fan noise to some extent, free-tipped fans offer certain advantages over banded fans, such as reduced material cost, reduced mass, and better balance. Thus, there is a need for a free-tipped fan which minimizes adverse performance effects presented by the lack of a tip band. In particular, there is a need for a fan which can develop the design blade loading in the presence of a tip gap. If a fan is designed without accounting for the gap, its actual loading will be different from the design loading, and the efficiency and noise performance of the fan will be compromised.